Frequency control devices are known to include temperature-compensated crystal oscillators (TCXO). A typical TCXO utilizes piezoelectric materials and temperature compensation circuitry to produce reliable oscillator output (e.g., high frequency waveforms) under varying environmental conditions. Such devices are commonly found in portable radio frequency (RF) communication equipment, such as cellular telephones. As consumer demand continually drives down the size of cellular telephones, the need for TCXO's having smaller dimensions becomes greater.
FIG. 1 illustrates a cross-sectional block diagram of a prior art TCXO (100) including a sealed piezoelectric element (102), temperature compensation circuitry (104), a substrate (106), input/output pads (107), and a device lid (108). The sealed piezoelectric element (102) includes a piezoelectric crystal (110), conductive adhesive (112), a crystal package (114), and a crystal package lid (116). Furthermore, a hermetic seal (118) disposed between the crystal package (114) and crystal package lid (116) creates an isolated crystal environment (120).
When a voltage is applied across the crystal (110), the crystal (110) resonates to produce the oscillator output. Also, the resonant frequency changes (i.e., drifts about a nominal frequency) responsive to changes in the temperature of the isolated crystal environment (120). A temperature sensing device (not shown) provides information to the temperature compensation circuitry (104) regarding the isolated crystal environment (120). As the temperature within the isolated crystal environment (120) fluctuates, the temperature compensation circuitry (104) modifies circuit parameters to ensure minimal frequency drift in the oscillator output.
Obviously, the height (124) and width (122) of the TCXO (100) are partially dependent upon the sealed piezoelectric element (102). That is, the finite thicknesses of the crystal package (114) and the crystal package lid (116) add to the overall dimensions of the TCXO (100). Typical dimensions (in millimeters) for the TCXO (100) shown are 11.4.times.13.97.times.3.58. While these dimensions may appear to be quite small, the demand for smaller cellular telephones necessitates TCXO's having even smaller physical dimensions. Therefore, a need exists for frequency control devices which overcome prior art dimensional limitations, but which meet the same performance criteria of their larger counterparts.